The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus of the type including a charging member that contacts or adjoins an image carrier.
A current trend in the image forming art is toward a miniature image forming apparatus in which a photoconductive element or image carrier, a developing device, a charger and so forth are constructed into a unit. A predominant type of charger includes a charging member implemented as a charge roller contacting the photoconductive element. A voltage is applied between the charge roller and the photoconductive element for uniformly charging the surface of the element.
The problem with the image forming apparatus of the type holding the charge roller in contact with the photoconductive drum is that toner left on the element after image transfer is apt to smear the charge roller. In light of this, sponge or similar cleaning pad is usually held in contact with the charge roller in order to clean the charge roller. The cleaning pad, however, has its cleaning ability lowered as the apparatus is operated over a long period of time, failing to remove deposits including toner and paper dust and other impurities from the charge roller. The deposits lower a discharging ability and thereby bring about background contamination. Background contamination ascribable to such deposits is more likely to occur when the charging member is implemented as a blade on which the cleaning pad cannot be mounted.
In order to solve the above-described problem, it is a common practice to execute, before a regular image forming operation, a prerotation mode operation for forming a pattern including a white portion and a gray portion on the photoconductive element. An optical sensor senses the reflection sensitivity of the white portion and that of the gray portion. A voltage to be applied to the charge roller is controlled on the basis of the resulting output of the sensor for thereby making up for the fall of the discharging ability and obviating background contamination.
The above conventional scheme, however, consumes extra power and extra time for causing the drum to rotate and causing a developing unit to operate to form the particular pattern before a regular image forming operation. This slows down the warm-up of the apparatus after the turn-on of a main switch or the recovery from a sleep mode.
A standard called ZESM (Zero Energy Standby Mode) has recently been proposed. In this respect, the warm-up time after the turn-on of a main switch or the recovery from a sleep mode should be further reduced.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 7-168420 and 11-95529.
It is an object of the present invention to provide an image forming apparatus capable of reducing the warm-up time.
In accordance with the present invention, an image forming apparatus includes charging member contacting or adjoining a photoconductive element for uniformly charging the surface of the element. A latent image forming device forms a latent image on the charged surface of the photoconductive element. A developing unit develops the latent image with toner by reversal development to thereby form a corresponding toner image. A temperature sensor is responsive to ambient temperature around the image forming apparatus. A voltage applying device applies a DC voltage to the charging member. A photosensor is responsive to the reflection density of the surface of the photoconductive element. A voltage control circuit varies the DC voltage in correspondence to the non-image area of the surface of the photoconductive element at preselected intervals, causes the photosensor to sense the deflection density of the surface of the element before and after the variation of the DC voltage, and controls a voltage to be applied from the voltage applying device to the charging member in accordance with a ratio between the resulting outputs of the photosensor. A decision circuit determines whether or not to cause the voltage control circuit to execute control over the voltage.
Also, in accordance with the present invention, an image forming apparatus includes charging member contacting or adjoining a photoconductive element for uniformly charging the surface of the element. A latent image forming device forms a latent image on the charged surface of the photoconductive element. A developing unit develops the latent image with toner by reversal development to thereby form a corresponding toner image. A temperature sensor is responsive to ambient temperature around the image forming apparatus. A voltage applying device applies a DC voltage to the charging member. A photosensor is responsive to the reflection density of the surface of the photoconductive element. A voltage control circuit varies the DC voltage in correspondence to the non-image area of the surface of the photoconductive element at preselected intervals, causes the photosensor to sense the deflection density of the surface of the element before and after the variation of the DC voltage, and controls a voltage to be applied from the voltage applying device to the charging member in accordance with a ratio between the resulting outputs of the photosensor. A decision circuit inhibits the voltage control circuit from executing control over the voltage for a preselected period of time necessary for the apparatus to be warmed up to a printing state, and then causes the voltage control circuit to execute the control on the elapse of the preselected period of time.